This invention relates generally to nuclear reactors and more particularly to a method of removing residual or decay heat from the core of a liquid metal cooled breeder reactor.
A nuclear reactor is designed and operated for the purpose of initiating and maintaining a nuclear fission chain reaction in a fissile material for the generation of heat, usually for power purposes. In the type of nuclear reactor described herein, fissile materials are contained within fuel rods or elements. A plurality of fuel elements or rods comprises a fuel assembly and a plurality of such assemblies comprises the heat generating nuclear reactor core which is structurally supported within a sealed pressure vessel. In a breeder reactor, the core also will include a fertile material contained in similar rods or elements which are combined to form an assembly. This fertile material, upon irradiation by fast neutrons, is converted to a fissile material suitable for use as fuel. A liquid metal coolant, such as liquid sodium or a mixture of sodium and potassium, is circulated into the reactor vessel and through the assemblies comprising the nuclear reactor core. There, the heat generated by nuclear fission is transferred from the fuel assemblies to the reactor coolant. The heated coolant exits from the pressure vessel and flows to a heat exchanger where the heat previously acquired is transferred by indirect heat exchange to another coolant coupled in sealing arrangement with the heat exchanger. The cooled liquid sodium exits from the heat exchanger and returns to a pump, where it is again circulated into the pressure vessel.
The system comprising the nuclear reactor core, reactor vessel, heat exchanger, circulating pump and the associated connecting piping is commonly referred to as the primary system. In a liquid metal cooled fast breeder reactor there generally is provided two or more coolant circulation loops in the primary system.
One of the accidents which must be guarded against in a nuclear reactor is a rupture of the connecting piping interconnecting a primary coolant pump and the reactor vessel. If the rupture is transverse to the axial centerline of the pipe, coolant will be discharged out of both ends of the ruptured pipe until the reactor is shut down and the pumps can be slowed down sufficiently so that no more coolant is being pumped through the ruptured pipes. Obviously, during this time, a considerable amount of coolant, normally supplied to the reactor core, is diverted out of the ruptured pipe and does not cool the core. This situation may cause extremely high core temperatures resulting in a failure of the fuel cladding and subsequent melting of the nuclear fuel contained within the core. It has been suggested that the entire system comprising the nuclear core, reactor vessel, heat exchanger, circulating pump, and connecting piping, all be placed within a housing and immersed in a pool of liquid coolant to minimize the risk of a loss-of-coolant accident.
Another potential hazard which must be guarded against is a complete loss of power. The nuclear reactors are designed such that in the unlikely event of a total power failure, the control rods will, nonetheless, automatically be reinserted into the core to shut the reactor down. The pumps have sufficient mass such that inertia of the rotating parts will continue to supply coolant long enough for full insertion of the control rods. However, even after the reactor is shut down by insertion of the control rods, the core will continue to produce heat, generally referred to as decay heat, even though the core is now subcritical. The decay heat is sufficient to produce temperatures that ultimately could melt the cladding around the fuel and perhaps even destroy the integrity of the pressure vessel.
Reactors generally are provided with an auxiliary power system, usually diesel-powered generators. However, upon starting a diesel engine, a finite amount of time is required to warm up the engine before a load can be placed upon it. Further, there is always the possibility that, for some reason, the engine will not start. Moreover, even after the diesel engine is started, the cooling pumps must be brought up to speed; and in view of the relatively large rotating masses involved, this can require a significant amount of time. During the time that the diesels are being started, warmed up, and the pumps being put back on line, the core is increasing in temperature. The temperature may be so great that fuel rod swelling and other deformation of the core could take place before the cooling system is returned to normal operation.
It has been proposed that if the reactor system is immersed in a large pool of liquid coolant, some means could be provided for cooling the core by convection currents. This, of course, requires some means for admitting the coolant from the pool into the pipes of the circulatory cooling system, either by the use of check valves or remotely controlled valves. However, these introduce yet another variable to the system in that the valves require moving parts and provide no assurance that they will work when needed. Thus, to increase the reliability of the system, redundancy is required, which, in turn, greatly enhances the cost and complexity of the system.
Obviously, there is need for an emergency core cooling system which could remove the decay heat from the reactor and which does not require the use of moving parts within the reactor vessel.